# Solar fuels and photosynthesis: Techniques

Three laboratory routes racing to outdo the humble leaf

Source page: https://www.materialinsights.org/energy/e_solar_fuels/techniques/

What this tab covers The methods used to capture this energy: how each works, and how they compare on efficiency, cost, life-cycle emissions, maturity and scale.

Photosynthesis is the only solar fuel route at global scale. Three artificial routes make hydrogen or hydrocarbons directly from sunlight, water and CO2; all are at research or pilot stage, competing with PV electricity feeding an electrolyser.

#### Comparison

| Method | Efficiency | Cost | Life-cycle emissions | Maturity | Scale |
|---|---|---|---|---|---|
| **Photosynthesis (agriculture and forestry)**^[1](https://experts.illinois.edu/en/publications/what-is-the-maximum-efficiency-with-which-photosynthesis-can-conv/)^[2](https://rogue.illinois.edu/sites/rogue.illinois.edu/files/2018-10/%234.pdf)^[6](https://ourworldindata.org/global-land-for-agriculture) | Maximum 4.6% (C3) or 6% (C4); soybean about 1.6% (calculated) | Not applicable (see the Biomass tile) | Removes CO2 while growing; net effect depends on land use (see the Biomass tile) | mature | Agriculture uses 48 million km2, 44% of habitable land |
| **Photoelectrochemical water splitting**^[3](https://doi.org/10.1021/acsenergylett.8b00920) | Record 19.3% | No commercial cost | No life-cycle figure | research | Laboratory devices |
| **Artificial photosynthesis (photocatalytic particles and CO2 reduction)**^[4](https://doi.org/10.1038/s41586-022-05399-1) | 9.2% indoors; 6.2% in a larger outdoor system | No commercial cost | No life-cycle figure | research to pilot | Outdoor prototypes only |
| **Solar thermochemical reactors**^[5](https://doi.org/10.1016/j.joule.2022.06.012) | 4.1% sunlight to syngas | No commercial cost | No life-cycle figure; carbon comes from captured CO2 | pilot | A 50 kW solar tower with 169 reflectors at IMDEA Energy, Spain, which ran the full chain to kerosene |

Photosynthesis is cheap and self-replicating but inefficient and land-hungry. The artificial routes could make fuel on land unsuitable for crops, but none is close to market: outdoor efficiencies of 4-6% are below a PV panel driving an electrolyser, and durability is unproven. The market is large but slow: almost 100 Mt of hydrogen was produced in 2024, less than 1% of it low-emission. Solar kerosene targets aviation, which has few other low-carbon options.

#### How each method works
**Photosynthesis (agriculture and forestry)**

Chlorophyll absorbs red and blue light and uses it to split water, releasing oxygen; the Calvin cycle then fixes CO2 into sugars, later built into starch, oil and wood.^[1](https://experts.illinois.edu/en/publications/what-is-the-maximum-efficiency-with-which-photosynthesis-can-conv/)^[2](https://rogue.illinois.edu/sites/rogue.illinois.edu/files/2018-10/%234.pdf)^[6](https://ourworldindata.org/global-land-for-agriculture)
**Photoelectrochemical water splitting**

A semiconductor electrode in water absorbs light and passes charges to catalysts (here rhodium) that split water into hydrogen and oxygen: a solar cell and electrolyser in one device, protected by a titanium dioxide coating.^[3](https://doi.org/10.1021/acsenergylett.8b00920)
**Artificial photosynthesis (photocatalytic particles and CO2 reduction)**

Light-absorbing semiconductor particles or nanowires (indium gallium nitride in the record device) carry catalysts and split water with no wires; related systems aim to reduce CO2 to fuels.^[4](https://doi.org/10.1038/s41586-022-05399-1)
**Solar thermochemical reactors**

Mirrors concentrate sunlight about 2,500 times onto ceria, which at about 1,500 C gives up oxygen; the cooled oxide then strips oxygen from water and CO2, leaving syngas that is made into kerosene by established processes.^[5](https://doi.org/10.1016/j.joule.2022.06.012)

#### Sources

1. [What is the maximum efficiency with which photosynthesis can convert solar energy into biomass? (Zhu, Long and Ort, Current Opinion in Biotechnology 19)](https://experts.illinois.edu/en/publications/what-is-the-maximum-efficiency-with-which-photosynthesis-can-conv/) University of Illinois Experts, 2008
2. [Improving Photosynthetic Efficiency for Greater Yield (Zhu, Long and Ort, Annual Review of Plant Biology 61, 235-261)](https://rogue.illinois.edu/sites/rogue.illinois.edu/files/2018-10/%234.pdf) University of Illinois (RIPE), 2010
3. [Monolithic Photoelectrochemical Device for Direct Water Splitting with 19% Efficiency (Cheng et al., ACS Energy Letters 3, 1795-1800)](https://doi.org/10.1021/acsenergylett.8b00920) American Chemical Society, 2018
4. [Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting (Zhou et al., Nature 613, 66-70)](https://doi.org/10.1038/s41586-022-05399-1) Nature, 2023
5. [A solar tower fuel plant for the thermochemical production of kerosene from H2O and CO2 (Zoller et al., Joule 6(7), 1606-1616)](https://doi.org/10.1016/j.joule.2022.06.012) Cell Press, 2022
6. [Half of the world's habitable land is used for agriculture](https://ourworldindata.org/global-land-for-agriculture) Our World in Data, 2024

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_solar_fuels/index.md)
- [Overview](https://www.materialinsights.org/energy/e_solar_fuels/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_solar_fuels/correlation/index.md)
- [Efficiency](https://www.materialinsights.org/energy/e_solar_fuels/efficiency/index.md)
- [Resources and labour](https://www.materialinsights.org/energy/e_solar_fuels/resources/index.md)
- [Impact on the biosphere](https://www.materialinsights.org/energy/e_solar_fuels/biosphere/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_solar_fuels/links/index.md)